The invention relates to methods for tuning an RF bandpass filter.
The methods according to the invention are intended to be used especially for tuning so-called combiner filters of a GSM system or a similar mobile phone network, but they can also be applied to bandpass filters suitable for other purposes, the frequency control of which filters requires a power measurement at selective frequency.
A combiner is a device by means of which many transmitters are connected to the same antenna or antenna line. Each radio transmitter is then connected to an antenna or antennae line via a separate bandpass filter, a so-called combiner filter. The medium frequency of each bandpass filter is tuned to the medium frequency of the respective radio transmitter. The object of the filters is, on the one hand, to input a transmission signal of a separate radio transmitter into the antenna at losses as low as possible and, on the other hand, to prevent as effectively as possible an entrance of transmission signals at different frequencies from the other radio transmitters into this separate radio transmitter from the antenna direction. Traditionally, combined filters have been tuned fixedly to the transmission frequencies of the respective radio transmitters. In such instances, it has not been possible to change the transmission frequency of a radio transmitter without changing the combiner filter or the tuning thereof at the same time.
However, it is often desirable to be able to change the frequencies of radio transmitters in a simple and quick manner. One such case is a base station of a cellular mobile phone system, for instance, with particular transmission and reception channels allocated for that base station. If the channel allocation of the system can be changed when required by changing the transmission and reception frequencies of the base stations, it is possible to utilize the channel capacity of the system flexibly and effectively under changing circumstances. For this reason, automatically tuned combiner filters have been developed, the medium frequency of which is automatically changed when the transmission frequency changes.
The tuning of these automatically tuned combiner filters is based on a measurement of RF power reflected from input of the filters or RF power passing through the combiner filters. The medium frequency of the filter is locked at a frequency at which the reflected power is at the minimum or the propagating power at the maximum.
In GSM and PCN mobile phone networks, a socalled GMSK (Gaussian Minimum Shift Keying) modulation serves as a method of modulating a transmitter signal. This modulation method is a relatively broadband one for signals corresponding to practical conditions, as a result of which it is difficult to tune the associated combiner filters each to a correct frequency and the tuning accuracy achieved remains rather modest.